1. Field of the Invention
The present invention relates to a variable optical attenuator, and particularly to an electrical variable optical attenuator having an integrated reflective device.
2. Description of Prior Art
Optical attenuators are widely used in optical transmission systems and optical networks. There are essentially two kinds of optical attenuators, variable and invariable. A variable optical attenuator is a passive optical component used to reduce optical power propagating in an optical fiber and to adjustably provide a range of amounts of attenuation. An invariable optical attenuator provides only one fixed setting of attenuation. Variable optical attenuators may be categorized as either electrical or manual. An electrical variable optical attenuator comprises an electrical controlling cell. Electrical variable optical attenuators are more widely used in optical transmission systems and optical networks, because they are more precise than manual variable optical attenuators.
A typical variable optical attenuator comprises a reflective mirror group having at least two independent reflective mirrors that form a light circuit.
Referring to FIG. 6, U.S. Pat. No. 6,292,616 discloses a variable optical attenuator comprising a U-frame, an attenuator plate 10, two independent mirrors 4, 5, two collimators 7, 8 and two optical fibers 6, 9. The U-frame is formed by folding a tubular member. A central axis of the U-frame is the optical axis of the attenuator. The attenuator plate 10 is disposed between the two mirrors 4, 5 in a central part of the U-frame.
Unfortunately, conventional variable optical attenuators such as that disclosed in U.S. Pat. No. 6,292,616 require very precise positioning of numerous optical components such as mirrors therein. This results in unduly high manufacturing and assembly costs. In addition, if optical components are imprecisely collimated, this often results in even more magnified imprecision in the amount of attenuation obtained. Thus the problem of unduly high manufacturing and assembly costs is exacerbated. Furthermore, if the conventional attenuator is subjected to vibration or shock during ordinary use, its mirrors are prone to shift position. This decreases the precision of attenuation.
Accordingly, an object of the present invention is to provide a variable optical attenuator which is easy to assemble.
Another object of the present invention is to provide a variable optical attenuator which ensures precise collimation of optical components therein.
A further object of the present invention is to provide a robust variable optical attenuator which resists vibration and shock.
To achieve the above objects, an electrical variable optical attenuator in accordance with the present invention comprises a housing, a cover, an optical module and a shifting device. The optical module comprises a reflective device, a graded transmittance filter, a filter carrier, and a container. The reflective device is an integrated piece comprising a first reflective plane, a second reflective plane, and an opening movably accommodating the graded transmittance filter therein. The first and the second reflective planes are substantially perpendicular to each other. Because the first and second reflective planes are integrally formed on the reflective device, the attenuator is relatively easy to assemble. In addition, the attenuator is able to operate reliably in rugged conditions, including applications where the attenuator may be subjected to vibration, shock or extreme temperatures.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: